surveying

surveyingPlane table surveying with contouring

Plane Table Surveying- Plane table is a graphical methodof surveying in which the field work and the plotting is done simultaneously.- It is adopted in small mapping.- It is also ideally suited to filling detail on a map already prepare and available on the drawing sheets.- It can also be used to prepare a fresh map with the linear measurements being taken with a chain or tape.

Plane Table And Accessories

Plane Table And TripodA simple plane table is a drawing board provided with a ball and socket arrangement for leveling the table ,with an arrangement to fix the table to a tripod.

Alidadea plane alidade consists of a metallic rule with a fine beveled edge called the fiduucial edge.This edge used for drawing line.For sighting-by placing the ruling edge along the points or lines marked on the sheet.Two frame are attached vertically at its end.The eye vane-a small metal frame with a silt.The object vane-a frame with a fine hair placed vertically ,for bisecting the part directed towards the object.The center of the eye vane and hair of the object vane provides the line of site.

Object VaneSight Vane

Fiducially Edge

Telescopic AlidadeThe alidade which is fitted with a telescope is known as a telescopic alidade. It is used to take inclined sights. It increases the range and accuracy of the sights. It consists of a small telescope with a level tube. A graduated scale is mounted on the horizontal axis.One side of the metal ruler is used as the working edge along which lines are drawn. The angles of elevation or depression can be read on the vertical circle.

Spirit LevelSpirit level can be used to level the plane table.It can be tubular level which can be placed in two perpendicular positions and leveled.

Trough CompassA compass is used for orienting the plane table when it has to be used in more than one station.When the table is shifted to another station ,the compass is placed along the meridian previously drawn and the table is rotated to make the needle read zero.The table is then oriented to the same position it occupied at the previous station.

Plumbing forkThe plumb bob is fitted on to a folded frame (fork).The straight edge can be placed on the table and the bottom leg on the frame is under the table.The top leg has a fine point which can be kept on a plotted point.

Plumbing Fork

Plane Tabling MethodsThe plane table can be used in four ways- i) radiation ii) intersection iii) traversing iv) resectionPlane table is generally used with the traverse drawn on a sheet for filling details orCan also be used for doing a fresh survey.These methods cover both these aspects of surveying with the plane table.

Radiation MethodIt requires the plane table to occupy a single station .Orientation table is not required.To conduct the survey of an area ,the table is kept at a convenient station P commanding a full view of the area to be surveyed.

Intersection MethodThis method requires setting the table up at minimum of two stations.Orientation is essential and be can done by back sighting.Two station A and B are selected so that they command a full view of the area to be surveyed.

Traversing MethodThis method is used to connect the two or more stations , however it is similar to the compass traversing but are done simultaneously.Traversing is a method of surveying where by a series of lines are surveyed.Traverse may be open or closed one.

Re-Section MethodThis is the method of orienting the table . a characteristic feature of resection is that the objective is to plot the station occupied by the table on the sheet rather than obtaining the other stations or plotting details.Two method of orienting the table Method of back sightingMethod using trough compass

Two-Point ProblemThis is a special case of resection to obtain the position of station occupied by the table ,given the plotted position of the two points.State to determine the position s of the station S occupied by the plane table, given the accurately plotted position ,p and q of the two points P and Q visible from the instrument station without occupying these two stations.

Three-Point ProblemThis is also a problem of resection.Objective is to obtain the position of the occupied the table after orientation.State given three visible stations and their plotted positions, to plot the station occupied by the plane table with the table correctly oriented with respect to the three points already plotted.

Advantage Of Plane Table SurveyingIt is suitable for location of details as well as contouring for large scale maps directly in the field.As surveying and plotting are done simultaneously in the field, chances of getting omission of any detail get less.The plotting details can immediately get compared with the actual objects present in the field. Thus errors as well as accuracy of the plot can be ascertained as the work progresses in the field.Contours and specific features can be represented and checked conveniently as the whole area is in view at the time of plotting.Only relevant details are located because the map is drawn as the survey progresses. Irrelevant details get omitted in the field itself.The plane table survey is generally more rapid and less costly than most other types of survey.As the instruments used are simple, not much skill for operation of instruments is required. This method of survey requires no field book.

Disadvantage Of Plane Table SurveyingThe plane table survey is not possible in unfavourable climates such as rain, fog etc.This method of survey is not very accurate and thus unsuitable for large scale or precise work.As no field book is maintained, plotting at different scale require full exercise.The method requires large amount of time to be spent in the field.Quality of the final map depends largely on the drafting capability of the surveyor.This method is effective in relatively open country where stations can be sighted easily .